Synthetic porous polymer structures display utility in a wide variety of industrial applications. Other uses include use as reverse osmosis membranes, filtration membranes, and insulating materials. The utility of a porous polymer structure for a particular application depends on a number of factors such as the pore size and pore size distribution, strength, and chemical composition of the structure. Porous polymer structures are prepared using a variety of techniques. However, there is still a need for new methods of preparation which will provide structures with unique properties. This invention provides a new method to prepare porous polymer structures and porous articles.
Porous polymer network structures may be characterized as either symmetric (isotropic), or asymmetric (anisotropic). In isotropic structures the pore size remains reasonably constant through the cross section of the structure while in anisotropic structures the pore size varies, usually from small to large, across the structure. Anisotropic structures have certain advantages for some types of applications such as resistance to fouling in membrane filtration. This invention relates to preparation of each structure type.
The two most widely used procedures for the preparation of porous polymer structures are Solvent Induced Phase Separation ("SIPS") and Termally Induced Phase Separation ("TIPS").
One example of SIPS involves dissolving a polymer in a solvent (solvent 1), applying the polymer solution to a smooth, non-porous surface to form a film, and then contacting the film with a second solvent (solvent 2) which is miscible with solvent 1 but does not dissolve the polymer. A solvent gradient is created at the interface where the polymer solution is in contact with solvent 2. This results in rapid polymer precipitation at the interface. As solvent 2 diffuses through the polymer solution, the gradient becomes less abrupt and polymer precipitation slows. Since pore size is generally related to the rate of polymer precipitation, the surface of the structure, where the polymer solution first comes into contact with solvent 2, contains the smallest pores. Pore sizes gradually increase across the remainder of the structure resulting in production of an asymmetric structure. It is very difficult to produce structures with uniform pore size (symmetric or isotropic) and with high strength using this technique.
TIPS relies on differences in polymer solubility in a solvent at high and low temperatures. To understand the process it is useful to refer to FIG. 1.